1. Field of the Invention
The invention relates to a communication terminal apparatus having compound functions of a G4 facsimile apparatus, a digital telephone, and the like and, more particularly, to a communication terminal apparatus which has an interface structure of nB+D (n.gtoreq.2, B: channel for transfer of user information, D: channel for transfer of signal information) and which is connected to a digital line network such as an ISDN (Integrated Service Digital Network) or the like.
2. Related Background Art
In recent years, the practical use of communication services by the ISDN has been started and attention is paid to various kinds of communication services using the ISDN. According to the ISDN, the high speed and high quality communication can be realized by using digital lines. It is expected that various communication services and communication terminal apparatuses using the ISDN will be developed and the network society and economy will be further promoted in future.
In the public telephone line network (PSTN) using conventional analog lines or the like, only one channel is provided for one line and, for instance, when a facsimile apparatus is used, the communication line must be used as a dedicated line for the facsimile apparatus or the facsimile apparatus and the telephone must be switched and used. In most such enterprises, the dedicated communication lines for the facsimile apparatus and the dedicated communication lines for the telephones are separately provided.
On the other hand, the ISDN has two user information transfer channels (hereinafter, referred to as "B channels") and one control information transfer channel (hereinafter, referred to as "D channel") even in the case where its interface is a fundamental interface. A communication terminal apparatus having compound functions of a facsimile apparatus and a telephone or other data communication terminal apparatuses or the like or a plurality of apparatuses can be connected to one communication line. The high speed and high quality communication can be perfomred as mentioned above. Further, in the data communication such as a facsimile communication or the like, a terminal apparatus having a re-calling function to again automatically call a line even when the line state is busy upon calling has been developed. A situation such that the user is detained is also reduced.
With respect to inventions regarding the ISDN, the assignee of the present invention has already filed U.S. Pat. No. 5,001,709, U.S. patent application Ser. No. 436,254 filed on Nov. 14, 1989, U.S. patent application Ser. No. 450,512 filed on Dec. 14, 1989, U.S. patent application Ser. No. 507,887 filed on Apr. 12, 1990, U.S. patent application Ser. No. 524,493 filed on May 17, 1990, and U.S. patent application Ser. No. 701,026 filed on May 16, 1991.
In the above communication terminal apparatus having the compound function, it is considered that the use frequency of the speech is generally largest and that the number of facsimile communications and other data communications is equal to at most tens of communications per day except in special cases. In general, a non-delay high response speed is not required in the facsimile communication or other data communication but a non-delay response speed is often required for the speech communication.
In the above conventional communication terminal apparatus, however, there is a problem such that when a line state is busy, although there is a terminal apparatus having a re-calling function as mentioned above, there is a case where even for a speech (or communication) which requires an emergency, the person who calls cannot execute a non-delay speech (or communication) for the called side until the busy state is released.